The usefulness of internal combustion engine pre-heaters (referred to generally herein as "engine heaters") is well established, particularly for use in vehicles. Preheating an engine prior to its being started in cold weather aids in starting the vehicle and extends the life of the engine, battery, starter and drive train components. As well, it lessens pollution, reduces fuel consumption, and hastens the delivery of heat to the windshield defroster and driver. Engine heaters, while used primarily in automobiles, may with little or no adaptation be used in other types of liquid-cooled internal combustion engines, including truck and recreational vehicle engines.
Conventional engine heaters generally rely on convection, conduction or vapour pressure to convey heat generated by a heating element to the engine. Conventional heaters generally fall into four categories: "block" (including "bolt-on block heaters"), "tank", "in-line", and "lower radiator hose". The names indicate their placement or application on the vehicle.
A block heater is fitted into the water jacket of an engine or, in the case of an aluminum block engine, may be bolted directly to the exterior of the engine block. In this latter case, much of the heat generated by the device is lost to the outside air. Block heaters consist of an electrical heating element housed within a sheath. The heat generated by the heating element is transferred to the engine largely by way of convection (in the case of water jacket placement) or conduction (in the case of external attachment). Due to the limited space available for these heaters, they are required to have a high "watt-density" ratio, which hastens their breakdown. As well, since convection moves the warmed fluid primarily in a vertical direction, considerable temperature differences within the engine may result. This may distort the engine and can confuse the thermal sensors within the engine sufficiently to prevent the engine from starting.
Installation of a block heater is difficult and expensive, and should be attempted only by a trained mechanic having the appropriate equipment. A further drawback of block heaters is their low position within the engine when typically installed in frost plug openings, which exposes the heater's electrical connections to salt, water, and other corrosive substances and can lead to a breakdown of electrical connections.
A tank heater is essentially a block heater within a housing through which engine coolant circulates by convection. The tank heater is typically installed externally to the engine, and is connected between a "T" junction in the lower radiator hose and a drain plug in the engine block. Hoses convey coolant to and from the device. Tank heaters have many of the drawbacks of block heaters, and suffer as well from their greater exposed surface area, which wastes heat. As well, since heat is transferred from the device by means of convection through the coolant hoses, heating of the engine may be unreliable.
A conventional in-line heater, represented by U.S. Pat. No. 3,626,148 (Woytowich et al.) is spliced into a hose of the passenger compartment heater circuit, and consists of a heating coil within a housing. The housing is provided with an inlet and an outlet, each having a one-way check valve conducting towards the engine water pump. The device is installed by cutting an existing hose of the engine and engaging the inlet and outlet, respectively, to the exposed hose ends. The heating coil vaporizes the engine coolant, and the hot vapour is expelled in pulses through the outlet valve, with the vapour condensing as it cools within the engine. The one-way valves are required to direct the flow of the expelled vapour. This device is typically bulky and requires the engine coolant to be a 50-50 glycol and water mix for optimal performance. As well, the device operates only at a relatively high temperature (about 250 degrees F.) in order to vaporize the fluid, and this results in reduced efficiency and heat losses from the exterior of the device. The device also requires the use of one-way valves, which impede the flow of fluid when the engine is running.
The lower radiator hose heater, as exemplified by U.S. Pat. Nos. 3,919,520 (Pickard) and 3,943,325 (Pickard), fits into the coolant hose between the radiator and the water pump. This type of heater is essentially a tank heater adapted to be installed in the radiator hose. However, since the engine thermostat blocks coolant flow between the engine and the radiator at temperatures below approximately 180 degrees F. (82 degrees C.), it is unlikely that a useful amount of heat can reach the engine with this device. As well, its proximity to the radiator will result in much of the heat generated by the device being dissipated without heating the engine.
The conventional heater designs described above generally rely on passive transmission of heat from the heater to the engine, for example by means of convection and conduction. The conventional in-line heater provides active circulation of heated fluid, but relies on heating the fluid substantially. As well, this device requires the use of one-way valves. By virtue of their reliance on a heat differential to heat an engine, conventional devices require the heater element to be relatively hot if the engine is to be heated within a reasonable time. This results in uneven heat distribution within the engine, and reduced efficiency of the heater.
The drawbacks described above may be addressed by an in-line type engine heater that actively pumps heated fluid through the engine by means of a pump driven by an electric motor. The use of a pump allows the heater to actively circulate a relatively large volume of fluid past its heater element, with the fluid being heated a relatively small amount with each cycle, thus improving efficiency and providing a more even distribution of heat throughout the engine than with conventional devices. There is no need to generate a large heat differential between the device and the engine. For example, a heater operating at 70 degrees F. will radiate heat from its exposed surfaces to the outside air at about one third the rate of a heater having the same surface area operating at 250 degrees F., with an ambient temperature of 0 degrees F. The positioning of such a device is not limited by thermodynamic considerations, and it may be installed in any convenient and protected location within the engine compartment. Ideally, the heater should as well be relatively small, in order to minimize heat losses from the surface of the device and to permit numerous placement options for installing the device. As well, it is desirable to provide the heater with a thermally-operated switch that turns the heater on only when engine temperature falls below a certain level or if engine coolant level within the heater is low.